gui: use PEP 440-compliant version number

Fixes this error from setuptools:

setuptools.extern.packaging.version.InvalidVersion: Invalid version: '2.10.1.Ludicrous.Lemur'

packaging commit 237ff3a ("Drop `LegacySpecifier` and `LegacyVersion`
(#407)") removed support for legacy versions. This was previously only a
warning from setuptools. This commit is included by packaging 22.0 and
setuptools 67.0.0.

Fixes #608

gui/setup.py

@@ -14,7 +14,8 @@ def read_version():
     with open('../.version', 'r') as handle:
         version_string = handle.read()
 
-    return version_string.strip()
+    version_numbers, _ = version_string.split(' ', 1)
+    return version_numbers
 
 
 def get_prefix():